1. Field of the Invention
The invention generally relates to fluoropolymers, and more particularly to methods of processing fluoropolymers which permit molding and extruding of materials which are not ordinarily moldable or extrudable, and to permit sintering fluoropolymers at lower temperatures. The invention has particular utility for processing polytetrafluoroethylene (PTFE).
2. Background of the Invention
Polymer technology has employed supercritical carbon dioxide as an alternative fluid medium to replace harmful organic solvents. The use of supercritical carbon dioxide in the synthesis of polymers is discussed in Cooper, A. I., J. Mater. Chem. 10:207 (2000); Ajzenberg et al., Chem. Eng. Technol. 23(10), 829 (2000), and U.S. Pat. Nos. 5,496,901 and 5,618,894 (both of which are herein incorporated by reference). These references describe solubilizing monomers, including fluoromonomers, in carbon dioxide, and then polymerizing the solubilized monomers to form polymers of interest. Supercritical carbon dioxide has also been used to extract low molecular weight components from polymer matrices as discussed in McHugh et al. Supercritical Fluid Extraction, Butterworth-Heimann, 1994. In addition, supercritical carbon dioxide has been used as a blowing agent for the production of polymer foams as discussed in Utracki et al. J. Polym. Sci. Part B-Polymer Physics 39(3), 342 (2001) and Cooper, ibid. Furthermore, supercritical carbon dioxide is used for polymer processing. See, Kwag et al., Ind. Eng. Chem. Res. 40(14), 3048 (2001) and Royer et al., J Polym. Sci: Polym. Physics 38(23), 3168 (2000). Coating applications require dissolution or suspension of polymer in solvent. Supercritical carbon dioxide has been used as a solubilizing and suspending media because of its benign nature and solvent characteristics as a function of temperature and pressure in the supercritical state. See, for example, U.S. Pat. Nos. 5,696,195, 6,034,170, and 6,248,823, all of which are herein incorporated by reference. There is a high affinity of amorphous fluoropolymers for supercritical carbon dioxide (see, Kazarian, J. Amer. Chem. Soc. 118(7), 1729 (1996). This may be due to interactions between carbon dioxide molecules in the supercritical phase and C═O and C—F bonds in the fluoropolymer. Semicrystalline fluoropolymers are usually only slightly swollen by supercritical carbon dioxide or dissolved at very high temperatures and pressures.
Fluoropolymers have superior chemical and solvent resistance characteristics, and excellent thermal stability. Because of these properties, fluoropolymers have been increasingly used in the chemical and semiconductor industry. However, processing of certain fluoropolymers, particularly PTFE and “modified” PTFE, can be difficult. For example, PTFE is not moldable or extrudable. Rather, PTFE components are typically cut or shaved from billets of material.
PTFE is often processed by techniques that resemble those for ceramics. PTFE is sintered at 370° C. before being formed by processes such as ram extrusion (see Scheirs, Modern Fluoropolymers, John Wiley & Sons, New York, 1997). Recently, fluoropolymer thermolysis (burning) and sintering has been identified as a potential source of halogenated organic acids in the environment (see Ellis et al., Nature 412:321–324 (2001)). These halo-acids, such as trifluoroacetic acid are persistent in the environment, as they have no known degradation process (see Boutonnet et al., Human and Ecological Risk Assessment 5:59–124 (1999)). Of more concern was the identification of long chain perfluorinated acids that accumulate in fatty tissue as carcinogens (see Upham, Int. J. Cancer 78:491–495 (1998)). In addition, chlorofluorocarbons (CFCs) were identified in the oxidation products of chlorofluoropolymers. These may migrate to the stratosphere and have a negative effect on the ozone layer. See Ritzvi, J. Thermal Analysis 45:1597–1604 (1995).
For some applications of fluoropolymers it is unnecessary and even undesirable to dissolve the polymer. Rather, it may be desired simply to swell or plasticize the fluoropolymer in order to change solid-state transition temperatures (melting and glass transition temperature), or solid-state structure. However, until this invention, very limited swelling of semicrystalline fluoropolymers with supercritical carbon dioxide has been observed. For example, Briscoe et al., J. Polym. Sci: Polym. Physics 29(989) (1991), reported that stiffer polymers like PTFE are difficult to expand and hence sorb less gas, and that the extend of carbon dioxide induced linear dilation is limited (approximately 0.2% at 42° C. and 3500 psi). Briscoe et al., J Polym. Sci: Polym. Physics 36(2435) (1998) reported that the maximum dilation for polyvinylidene fluoride at 4400 psi is 1.2% at 80° C. and 0.85% at 42° C. Aurora et al., Macromolecules 32:2562–2568 (1999) and Kung et al., Macromolecules 33:8192–8199 (2000) discuss the manufacture of polymer blends (e.g., PTFE-co-hexafluoropropylene (FEP)/polystyrene) by infusing a carbon dioxide solution carrying monomers into the FEP. Aurora et al. report 4.4% carbon dioxide incorporation for swelling of FEP at 80° C. and 3530 PSI.
After synthesis, PTFE and modified PTFE are sintered prior to other processing. The manufacture of polymer products by sintering of polymers in the powder form is well known. This technology is employed for processing of polymers where conventional techniques such as extrusion, injection molding, etc., are either non-economical or difficult to utilize due to the extremely high viscosities of the polymer melt. For example, see Ebnesajjd, S. Non-Melt Processable Fluoroplastics: The Definitive User's Guide and Databook, PDL Handbook series, Norwich, N.Y., 2000 and U.S. Pat. No. 4,064,077 which is herein incorporated by reference.
Generally, there are two types of sintering processes:
In the first process, the powder is compressed or compacted by subjecting it to high static pressure. Then sintering of the compressed preform is effected by means of thermal treatment at temperatures greater than the melting point or softening point of the polymer.
The latter treatment brings about the coalescence of individual polymer particles and reduces or eliminates voids. This is called “free sintering” since sintering is carried out without the application of pressure (Ebnesajjd, ibid.)
In the second process, the powder is compressed or compacted by subjecting it to high static pressure. After only partial pressure release, the preform is heated to the sintering temperature. A post-thermal treatment is then provided to eliminate internal stresses. This is called “pressure sintering” (Ebnesajjd, ibid., and U.S. Pat. No. 6,066,280 which is herein incorporated by reference).
In spite of the popularity of these processing techniques for polymers such as PTFE, UHWMPE, etc., these processes have significant drawbacks and special care must be taken to obtain a homogenous product with good mechanical properties such as tensile strength. Convention sintering and pressure sintering have the following disadvantages:
a) Polymers are generally good thermal insulators (e.g., the thermal conductivity of PTFE is approximately 0.25 W/m°K). This impacts the rate of sintering of the preform. Traditionally, the preform is heated by circulating large quantities of hot air. The design of the oven is thus critical to prevent the formation of hot spots in the preform.
b) Though the economics of the sintering process favors high heating rates, the low thermal conductivity of the preform can result in the formation of thermal gradients between the outside and insider of the preform leading to cracks in the preform. The size of the preform thus dictates the maximum permitted heating rates which may be quite low.
c) As for the heating process, controlled cooling of the sintered product is critical to obtain a homogenous crack-free product. In general, hold periods are introduced during the heating and cooling cycle to allow the heating or cooling of the interior section of the preform, thereby reducing the thermal gradient, and hence the possibility of crack formation. This results in much longer times for the sintering process which translates into increased manufacturing costs.
d) The temperature of sintering is usually 20–30° C. above the melting point of the polymer. For example, in the case of PTFE, the sintering temperature ranges from 360° C. to 380° C. (Ebnesajjad, ibid.). At this temperature, the adjacent melted PTFE particles fuse together and coalesce, thereby reducing voids. However, due to extremely high melt viscosity (e.g., approximately 1011 to 1012 poise for PTFE) molecular mobility is severely inhibited. To compensate for this reduced mobility, long sintering times are required depending on the size of the product. However, maintaining the polymer above its melting temperature for long periods of time can result in degradation that leads to product contamination or diminution in properties. This is a problem particularly for demanding applications such as microelectronics fabrication operations. Furthermore, the decomposition products may be environmentally harmful as noted above (see Ellis et al., Nature 412:321–324 (2001)).